Platinum temperature sensor and its method of production

ABSTRACT

A Platinum temperature sensor comprises a ceramic substrate and a platinum thin-film resistor applied to said ceramic substrate, a ceramic cover layer and a connecting layer generated from a ceramic green layer by pressure and temperature treatment. The ceramic cover layer is connected with the ceramic substrate in such a way via the connecting layer that the platinum thin-film resistor is sealingly encapsulated with regard to the environment.

PCT/EP00/00178 filed on Jan. 12, 2000, with the European Patent Office.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a platinum temperature sensor and amethod for producing the same, and in particular to a platinumtemperature sensor in the case of which a platinum thin-film resistor,which is applied to a ceramic substrate, is used for temperaturedetection.

2. Description of Prior Art

A known platinum temperature sensor is shown in FIG. 3. In this knownplatinum temperature sensor, a platinum thin-film resistor 2 is appliedto a ceramic substrate 4 which normally consists of aluminium oxideAl₂O₃. In the area in which the platinum thin-film resistor 2 is formed,a protective glaze 6 is provided on the surface of the ceramic substrate4. The platinum layer, in which the platinum thin-film resistor 2 isnormally formed in a meandering shape, is additionally patterned so asto include connecting areas 8 having lead wires 10 connected thereto inan electrically conductive manner for taking the sensor signal. Forfixing the lead wires 10, a glaze 12 is provided.

The field of use of the platinum temperature sensor, which is shown inFIG. 3 and which is implemented in thin-film technology, is normallylimited to 600° C. In the last few years, there has, however, been anincreasing demand for an embodiment that can be used for higheroperating temperatures, which may exceed 1,000° C. In the field ofhigh-temperature sensors considerable efforts have therefore been madeto provide platinum temperature sensors which are suitable to be used insuch high temperature ranges. By purposefully selecting the compositionof the protective glaze 6, it has already been possible to findsatisfactory solutions for some cases of use, whereas in very particularfields of application, e.g. in special cases of use in the field ofautomotive engineering, the results do not satisfy all requirements. Forexample, the long-term stability of temperature sensors of the typedescribed hereinbefore, especially when they have applied thereto acertain measurement current, which may e.g. be 5 mA, is not sufficientlyguaranteed at the high temperatures occurring, viz. temperatures in therange of 800° C. and 1,000° C., since the protective glazes used may bedecomposed electrochemically by the necessary measurement current atthese high temperatures. The resultant material migration has a negativeinfluence on the properties of the platinum so that the stability of thesensors and, consequently, the measuring accuracy are impaired.

By purposefully selecting the composition of the protective glazes,improvements could be achieved to a certain extent, but it wasimpossible to find protective glazes that withstand the electrochemicaldecomposition by the measurement current in the case of continuous loadsin a temperature range of 1,000° C. or more than 1,000° C.

From the article “Fügen von Technischen Keramiken mitKeramik-Grünfolien” by M. Neuhäuser et al., sfi/Ber. DKG 72 (1995) Nr.1-2, methods for joining technical ceramics are known wherein ceramicsgreen foils are used to connect two ceramic layers. A prerequisite forthe joining method described there is that the sintering temperature ofthe ceramic green foil is below the sintering temperature of the ceramicto be jointed.

A temperature sensor having a platinum resistance layer, which isapplied to a ceramic substrate and encapsulated by a glaze, is disclosedin DE 7629727 U1.

From De 37 33 192 C1 a PTC-temperature sensor is known herein a platinumresistor formed by means of a platinum thick-film technique is arrangedbetween two ceramic green foils and an interlaminar binder layer,whereupon the two foils are laminated together by use of a pressure anda risen temperature and are sintered after that.

In DE 4445243 A1 a temperature sensor is described wherein threeunprocessed ceramic substrates are laminated together, pressed and firedat 1.600° C. in order to form a uniform piece. Before laminating aplatinum resistor is arranged between two of the ceramic substrates.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide a platinumtemperature sensor which supplies reliable measurement results even inthe case of continuous loads in a high temperature range, and a methodfor producing such a platinum temperature sensor.

According to a first aspect of the invention this object is achieved bya platinum temperature sensor comprising:

a ceramic substrate;

a platinum thin-film resistor applied to the ceramic substrate;

a ceramic cover layer; and

a connecting layer produced from a ceramic green sheet by pressure andtemperature treatment, by means of which the ceramic cover layer isconnected with the ceramic substrate in such a way that the platinumthin-film resistor is sealingly encapsulated with regard to theenvironment.

According to a second aspect of the invention this object is achieved bya platinum temperature sensor comprising:

a ceramic substrate;

a platinum thin-film resistor applied to the ceramic substrate;

a ceramic cover layer; and

a connecting layer made of a glaze that is applied to the ceramicsubstrate in a boarder area surrounding the platinum thin-film resistorby means of which the ceramic cover layer is connected with the ceramicsubstrate in such way that the platinum thin-film resistor is sealinglyencapsulated with regard to the environment.

According to a third aspect of the invention, this object is achieved bya method for producing a platinum temperature sensor comprising thesteps of:

providing a fired ceramic substrate with a platinum thin-film resistorapplied to the main surface thereof;

applying a connecting layer made of a ceramic green layer to the mainsurface of the ceramic substrate; and

applying a fired ceramic cover layer to the connecting layer in such away that the platinum thin-film resistor is sealingly encapsulated withregard to the environment by subjecting the ceramic green layer to atemperature treatment under application of pressure in such a way thatthe ceramic substrate and the ceramic cover layer will be connected.

The present invention is based on the knowledge that ceramic materials,especially aluminium oxide Al₂O₃ are insensitive to the above-describedcurrent induced decomposition, and thus this material that in additionis used in thin-film platinum temperature sensors as substrate materialfor the platinum film can advantageously also be used as protectivematerial for capsulation of the platinum film layer. Thus, the problemsof electrochemical decomposition and the connected deterioration ofproperties of the platinum temperature sensors are prevented, even whenthe protective cover is realized from a ceramic material.

On the one hand, the structured platinum film in the inventive platinumtemperature sensor is therefore sufficiently protected againstmechanical and chemical environmental influences. On the other hand theinventive platinum temperature sensor supplies reliable measurementresults even in the case of continuous loads in a high temperature rangeof for example 1000° C. or more than 1000° C., since the above-describeddisadvantageous decomposition phenomenons do not occur in the case ofthe inventive platinum film temperature sensor.

In the inventive platinum temperature sensor the connecting layer iseither applied to the whole area of the ceramic substrate provided withthe platinum thin-film resistor or alternatively only on a border areaof the same, so that the platinum thin-film resistor is surrounded bythe connecting layer. If the connecting layer is only provided on theborder area it is preferable to provide a sealing layer that can be madeof glass, for example, on the side edges of the resulting layerstructure. However, such a layer can also be provided when theconnecting layer is applied to the whole area.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will be explained belowin detail referring to the enclosed drawings. They show:

FIG. 1: a schematic cross-sectional view of a platinum temperaturesensor according to the present invention;

FIG. 2: a schematic top view of a platinum temperature sensor accordingto the present invention; and

FIG. 3: a schematic cross sectional view of a known platinum temperaturesensor.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

First, referring to FIG. 1, a first embodiment of a platinum temperaturesensor according to the present invention is described in detail. Theplatinum temperature sensor comprises a ceramic substrate that consistsof Al₂O₃ in preferred embodiments on which a structured platinum layeris applied defining a platinum resistor trace 2 and connecting areas 8.A lead wire 10 can be electrically conductive connected with theconnecting areas 8. A ceramic cover layer 16 which is also made fromAl₂O₃ in the preferred embodiments of the present invention is appliedon the surface of the ceramic substrate 4 on which the platinum resistortrace is provided via a connecting layer 14, which consists also ofAl₂O₃ in the preferred embodiments of the present invention. Further, inFIG. 1 a glass ceramic 18 is illustrated that serves for fixing the leadwire.

It is preferable to use the same material for the cover layer 16 that isused for the ceramic substrate 4. However, a similar material can beused. The cover layer 16 can have the same thickness as the ceramiclayer or a smaller thickness, for example 10 to 20% thinner, as long asthe thickness of the cover layer can obtain a sufficient protectiveeffect against adverse environmental influences. The connecting layercan have a small thickness since the protective effect for the platinumfilm with regard to the outer atmosphere is achieved by the thickercover layer 16.

First, for producing the platinum temperature sensor according to theinvention the fired ceramic substrate 4 that is a Al₂O₃-substrate in thepreferred embodiments of the present invention is supplied with theplatinum resistor trace 2 provided thereon. Subsequently, a very thinceramic green layer with a thickness of for example 0.1 to 0.2 mm issupplied at least in the area of the platinum resistor trace 2. Thisconnecting layer is preferably an Al₂O₃ green layer. Subsequently thefired cover layer 16, preferably also a Al₂O₃ layer, is applied to thegreen layer. This total layer construction will then be fired with veryhigh temperatures under an additional outer pressure that can forexample be generated by an additional ceramic plate. By this firingprocedure the two ceramic plates, i.e. the ceramic substrate 14 and thecover layer 16 are tightly connected via the foil 14. Thus, the platinumthin-film resistor 2 is tightly closed against the outer atmosphere.

Thus, the present invention provides a platinum temperature sensor thatensures a secure protection of the platinum thin-film resistor againstpenetration of foreign material from the environment even in hightemperature ranges, wherein no chemical decomposition influences by themeasurement current necessary for operating the temperature sensor needto be feared.

It is not possible to apply the ceramic cover layer directly to theceramic substrate by means of screen-printing, like it is done whenusing glazes as protective layer, for example. IF Al₂O₃ is used as acover layer this procedure cannot be applied since the melting point forAl₂O₃ is much to high, namely above the melting of the platinum andwould further also melt the carrier substrate.

Even when the whole cover layer is applied as a green layer, whereingreen layer means a ceramic with binding agents volatilizing in thefiring procedure, problems arise since this green layer is subject to acertain shrinkage, wherein the cover layer would not have the requiredimpermeability due to shrinkage cracks. The above-mentioned shrinkagebecomes even more apparent the thicker the foil is. On the other hand, acertain thickness in the range of 0.3 to 0.5 mm is necessary in order toachieve a sufficient protective effect against adverse environmentalinfluences with the mentioned high temperatures of 800° C. to 1000° C.or more.

In FIG. 2 a schematic top-view of an alternative embodiment of Aplatinum temperature sensor according to the invention is illustrated.In FIG. 2 again the ceramic substrate 4 servicing as a carrier and theplatinum layer structured thereon that defines a platinum thin-filmresistor 2 and connecting areas 8 shown. Further two lead wires 10 eachconnected with one of the connecting areas 8 are illustrated. However,in the embodiment shown in FIG. 2 the connecting layer 10 is not appliedto the whole area of the surface of the ceramic substrate 4 and theplatinum thin-film resistor 2 but only in a border area surrounding theplatinum thin-film resistor 2. In this embodiment the platinum thin-filmresistor is not coveted by the connecting layer. By means of thisconnecting layer 20 surrounding the platinum thin-film resistor 2 acover layer (not shown in FIG. 2) ill now be connected with the ceramicsubstrate 4, so that again the platinum thin-film resistor 2 issealingly encapsulated with regard to the environment. In thisembodiment it is preferred to provide an additional sealing layer madeof glass, for example, on the perpendicular outer edges formed by thelayer structure.

In the embodiment shown in FIG. 2 either also a Al₂O₃-foil can be usedas a connecting layer 20, while alternatively a protective glaze forsealing connection, i.e. fusion of the carrier substrate with the coverfoil, can be used a connecting layer instead of the foil. Even when theprotective glaze is used a connecting layer 20 the current carryingplatinum resistor trace 2 has no contact to the glaze layer 20, so thatthe above-described decomposition phenomenons do not occur in this case.Using a glaze for the connecting layer 20 can offer advantages for theproduction and can be used for up to 1000° C. if a high melting glazewith a melting temperature of more than 1.300° C. is used.

The connecting layers 14 in FIG. 1 and 20 in FIG. 2 used according tothe invention can alternatively be replaced by a ceramic paste printedon by a screen-printing method. In this case a ceramic paste is printedonto the structured ceramic substrate coated with the structuredplatinum film by screen-printing and covered with the ceramic coverplate after pre-drying. Subsequently under application of pressure afiring is performed in order to connect ceramic substrate and ceramiccover layer. The ceramic paste can consist of a pasted mixture ofseveral ceramic powders and quartz powders, for example Al₂O₃, MGO,SiO₂.

Thus, the present invention provides a platinum temperature sensor,which on the one hand provides a secure protection of the platinum filmresistor against outer influences and on the other hand exhibits nodeterioration of precision even in the case of continuous loads in ahigh temperature range.

The platinum temperature sensor according to the invention canpreferably be produced by wafer processing, with the exception ofmounting the lead wires and fixing them in such a way that a pluralityof platinum temperature sensors can be produced from one ceramic wafer.After dicing-up the individual platinum temperature sensors theabove-mentioned sealing layer can be applied to the respectiveperpendicular cutting edges.

What is claimed is:
 1. Platinum temperature sensor comprising: a firedceramic substrate; a platinum thin-film resistor applied to the ceramicsubstrate; a fired ceramic cover layer; and a connecting layer by meansof which the fired ceramic cover layer is connected with the firedceramic substrate in such a way that the platinum thin-film resistor issealingly encapsulated with regard to the environment, wherein theconnecting layer is produced by arranging a ceramic green sheet betweenthe fired ceramic cover layer and the fired ceramic substrate and bysubjecting the ceramic green sheet to a temperature treatment underapplication of pressure.
 2. Platinum temperature sensor according toclaim 1 wherein the connecting layer is formed as a continuous face onthe ceramic substrate and the platinum thin-film resistor.
 3. Platinumtemperature sensor according to claim 1 wherein the connecting layer isapplied in a frame-like shape in a border area surrounding the platinumthin-film resistor on the ceramic substrate.
 4. Platinum temperaturesensor according to claim 1 wherein the connecting layer is generatedfrom an Al₂O₃ green-layer.
 5. Platinum temperature sensor according toclaim 1 wherein the ceramic substrate is made of Al₂O₃.
 6. Platinumtemperature sensor according to claim 1 wherein the ceramic cover layeris made of Al₂O₃.
 7. Platinum temperature sensor according to claim 1wherein a sealing cover is applied to the outer peripheral edges of thelayer structure consisting of ceramic substrate, connecting layer andceramic cover layer.
 8. Platinum temperature sensor according to claim 7wherein the sealing cover is made of glass.